Liquid crystal displays have the advantage of requiring less power than cathode ray tubes or arrays of light emitting diodes. Therefore liquid crystal displays have become popular for use when low power is important. Since low power is a desirable parameter, a further reduction in power required to operate a liquid crystal display is also desirable.
A liquid crystal display is divided into separate picture elements or pixels each of which can be separately controlled to present an ON color or an OFF color. These separate pixels are often arranged in a rectangular array, as shown for example in FIG. 1. A first voltage is applied by a row driver to one side of the liquid crystal material forming the pixel and a second voltage is applied by a column driver to the other side of the liquid crystal material forming the pixel. As shown in FIG. 1 in exploded view, parallel rows 1 through n of conductive lines Rl through Rn are driven from row drivers RDl through RDn (not shown). These row conductive lines are located adjacent to one face of liquid crystal LQll. Parallel columns l through m of conductive lines Cl through Cm are located adjacent to the other face of liquid crystal LQll. Liquid crystal LQll comprises pixels Pl,l through Pm,n arranged in a rectangular array. The numbering system used here identifies a pixel by the number of the row above it and the column below it. Thus pixel P3,2 is between lines R3 and C2. The liquid crystal material of the pixel responds to the integrated RMS voltage difference between its row and column lines by becoming absorbent at voltages on one side of a transition voltage and transmissive at voltages on the other side of the transition voltage.
The earliest liquid crystal displays used a control system which simply applied a first voltage across a pixel to produce the ON color and a second voltage across the pixel to produce the OFF color. More recently, the control has been multiplexed so that the control voltage to any one pixel was applied only part of the time, a background voltage being applied for the remainder of the time. Multiplexing requires more complex control methods in which more than two control voltages are generated.
Until recently, the twisted nematic material of the liquid crystal required a voltage difference on the order of 10-15% to achieve good contrast between the ON state and the OFF state. FIG. 2 shows a comparison between optical and voltage characteristics of older and newer materials, the newer materials showing a change in reflectivity over almost the full range within a voltage range of 5-10% of the total voltage, or within a voltage difference of some 0.1 to 0.2 volts in a system using liquid crystal material with a 2 volt threshold. The OFF voltage is maintained just below the highest voltage at which the appearance of the crystal is acceptably OFF. This is called the threshold voltage. The ON voltage is maintained at slightly more than the voltage at which the appearance of the crystal is acceptably ON. The difference between the ON and OFF voltages is called the transition voltage. The new materials have a transition voltage as low as 5% of the threshold voltage. This new material is 180.degree. to 270.degree. twisted nematic liquid crystal material and is available from Hoffman LaRoche. Because the newer material is so sensitive, it is possible to adequately control a pixel of a display by applying a signal with a duty cycle as small as 1/256. That is, a single column driver can serially control as many as 256 pixels in one column.
Liquid crystal displays are typically driven by selecting a particular row of pixels and activating all columns in that row simultaneously. Such a system was developed as a result of studies by P. Alt & P. Pleshko, described in a paper entitled "Scanning Limitations of Liquid Crystal Displays", IEEE Trans. Electron Devices, Vol. ED-21, No. 2, PP 146-155, 1974, which is incorporated herein by reference. A display having 640 columns and 200 rows can be driven by applying a high voltage row select signal to each pixel in the display 1/200 of the time. The other 199/200 of the time, each pixel sees a bias voltage of slightly less than the threshold voltage. Displays having more than 256 rows can be driven by dividing the display into portions and driving each portion separately, to avoid having a duty cycle lower than 1/256. However, dividing into portions requires providing additional overhead circuitry for driving the additional portions, with consequent increased cost and power usage.
Liquid crystal material must be driven with a net DC voltage of zero in order not to damage the crystal. Various methods have been used to reverse polarity of the voltage in order to achieve a net DC voltage of zero. In one method, the entire frame of the display is scanned applying voltage of one polarity, then the polarity is reversed for an identical scan.
FIG. 3 shows a timing diagram for a portion of a display driven by this method. The voltages applied by the row 2 driver and column 1 driver during the row 2 select time (row 2 is selected) and during other nearby times (the row 1, row 3-5, and row n select times, where n is the last row) are shown. Two sections of the timing diagram are shown in order to demonstrate phase 1 and phase 2. During phase 2 the information of phase 1 is repeated but in opposite polarity, in order to achieve the resultant DC voltage to the pixels of zero. As shown in FIG. 3, during phase 1 for the row 1 select time, the row 2 driver applies an "unselected" voltage of V4, in one embodiment -14.4 volts. At row 2 select time the row 2 driver applies a voltage of V0, in this embodiment 0 volts. For row 3 select time to row n select time the voltage applied by the row 2 driver is again V4. The system then moves to phase 2. During row 1 select time, the row 2 driver applies a voltage V1, in this embodiment -1.6 volts. At row 2 select time the row 2 driver applies a voltage of V5, in this embodiment -16 volts. Subsequently the row 2 driver returns to V1 or -1.6 volts for the remainder of phase 2.
During phase 1, the column 1 driver applies a voltage V3 or V5, which is 1.6 volts either side of the V4 voltage applied by the row drivers when not selected. Thus when not selected a pixel receives the 1.6-volt difference. When selected, which in FIG. 3 for row 2 is during the row 2 select time, the pixel sees a larger voltage difference between its row line and its column line, in the above example 16 volts for an ON pixel. During phase 2, for this ON pixel, row 2 sees a voltage of -1.6 volts when not selected and -16 volts when selected. The column 1 driver applies voltages of 0 or -3.2 volts during phase 2 depending on the intended state of pixels in column 1. The voltage waveform experienced by the pixel in row 2 column 1 is also shown in FIG. 3. The pixel in row 2 column 1 sees a voltage difference of +1.6 or -1.6 volts when not selected and, because it is to be an ON pixel, + 16 or -16 volts when selected. Since the polarity of all voltages experienced by the pixels is reversed during phase 2, net DC voltage is approximately zero. Thus, this method is satisfactory for maintaining the life of the crystal. However, the current to the display drivers, which depends on the frequency of voltage reversal of both row and column drivers, can vary over a tremendously wide range. Displays are scanned a minimum of 50 times (25 pairs) per second to avoid any flicker visible to the human eye. Therefore frequency of voltage reversal cycles can vary from a low of 25 Hz in the case when all pixels are the same color to a high of 5,000 Hz in the case of a checkerboard pattern where phase reversal in one column occurs every pixel for 200 rows. Such a 200:1 frequency variation and therefore current variation has resulted in an inefficient driving mechanism. The difference in polarity reversal frequency seen by the pixels may also reduce the contrast of the display, as will be discussed later. This reduced contrast is most commonly caused by a change in RMS voltage seen by the pixels caused by the fact that the rounded corners of the square wave representing pixel voltage contribute to reducing the RMS voltage more at high frequencies than at low frequencies. The threshold voltage of the crystal increases somewhat with frequency, thereby causing further reduced contrast due to difference in polarity reversal frequency seen by different pixels.
For a battery driven circuit, the single DC voltage of the battery must be converted to a plurality of voltages to drive the multiplexing circuits which control the display. A switching regulator is frequently used for this purpose. FIG. 6 shows such a prior art switching regulator combined with a voltage divider chain with operational amplifiers. The battery provides the voltage difference between Vcc and ground. A first end of a primary coil P61 is connected to one terminal, in this case Vcc, of the battery. The other end of primary coil P61 is connected through switching transistor T61 to ground (the other terminal of the battery). Power delivered by the battery is determined by controlling the on-time of switching transistor T61. A higher on-time produces a higher peak current through primary P61 and a corresponding higher delivery of power to secondary coil S61. Diode D61 and capacitor C61 form a loop with secondary S61. When secondary S61 is driven in a first direction, current flows through diode D61 and charge builds on capacitor C61. When secondary S61 is driven in the opposite direction, current can not flow through diode D61, so built up charge remains on capacitor C61. Thus capacitor C61 supplies a voltage for in turn generating multiple voltages, in this case six voltages, which supply multiplexing circuits M6 for controlling a liquid crystal display.
Typical voltages used to drive the multiplexing circuits are, for example, 0 volts, 1.6 volts, 3.2 volts, 12.8 volts, 14.4 volts, and 16 volts. These six voltages can provide a high voltage difference (16 volts) for driving a selected row line and a low voltage difference for applying data to columns. These six voltages allow a voltage difference of +/-1.6 volts to be applied to each column in each unselected row while for the selected row they allow a voltage difference of 12.8 volts to be applied to all OFF pixels and 16 volts to be applied to all ON pixels. A resultant zero DC voltage across each pixel is also provided. The circuit of FIG. 6 can provide the above set of voltages by providing resistors R62 through R66 proportional to the desired voltage differences, as is well known. Charge proportional to the values of resistors R62 through R66 are stored on capacitors C62 through C66 respectively.
To avoid drawing current high enough to alter the relative voltages on the capacitors, operational amplifiers OA62 through OA65 receive the voltage present on plates of capacitors C62 through C65 respectively, and provide amplified output signals having voltages V1 through V4. An operational amplifier may also amplify the current drawn from the low voltage plate of capacitor C66 and provide the amplified signal on V5. Alternatively, as shown in FIG. 6, follower transistor T66 provides a voltage on line V5 higher by the base-emitter voltage drop of transistor T66 than the variable voltage level of resistor R67.
This voltage divider operational amplifier chain is not efficient in use of power. In the FIG. 6 example, to generate a 1.6 volt output pulse from one of the operational amplifiers requires sourcing current to drive the operational amplifier from a 16-volt supply. Thus the operational amplifiers dissipate considerable power. One voltage divider chain typically requires 30-300 milliwatts of power, which in a typical 200 to 1 multiplexed display is 10 to 100 times that actually needed by the capacitive load of the display.
The circuit of FIG. 6 can supply the six voltage levels shown in FIG. 3. Using the method of FIG. 3, the pixels will receive an average DC voltage close to zero. Because resistors determine the relative voltages, the variation in resistance values causes the net DC current to vary somewhat from zero and thus shorten the life of the liquid crystal.
As shown in FIGS. 4a-4c, another method of driving the display described by J. R. Hughes, in a paper entitled "Contrast Variations in High-Level Multiplexed Twisted Nematic Liquid-Crystal Displays", IEE Proceedings, Vol. 133, No. 4, August 1986, reverses the polarity of a pulse twice for every row select time so that for the first half of a period in which a row is activated (selected) the voltage applied to a row driver is a first polarity and for the second half of the period the voltage is the opposite polarity. The Hughes paper is incorporated herein by reference. With the Hughes method for scanning a display having 200 rows of pixels at a rate of 50 scans per second, one column driver in a display in which all pixels are one color changes the applied voltage twice per row. As the number of reversals increases the driving frequency decreases. This can be seen by looking at the pixel waveforms in FIGS. 4a-4c. FIG. 4a shows a bit map of a small portion of a display. As shown in FIG. 4a, pixels in row 2 col. 1 and row 4 column 1 are to be colored dark (OFF) and other pixels are to be light (ON). Thus the pixels in column 1 are to be of an alternating pattern while the pixels of column 2 are to be of one color. FIG. 4b shows the waveforms for rows 1-3 and columns 1-2 of the display. During the first half of row 1 select time, row 1 receives a high voltage, and during the second half of row 1 select time row 1 receives a low voltage. These voltages may be, for example, +14.4 volts and -14.4 volts and may be obtained from the voltage divider circuit of FIG. 6 by appropriately connecting lines V0 through V5 to the display. Row 2 select time is an unselected time for row 1. During unselected times row 1 receives a zero volt signal. During row 2 select time row 2 receives first a high and then a low voltage, and during row 3 select time row 3 also receives first a high and then a low voltage. Each row is thus activated with a high voltage for the first half of the select time followed by a low voltage for the second half of the select time.
To produce an ON pixel at row 1 column 1, during the first half of row 1 select time column 1 presents a low voltage, for example -1.6 volts, and during the second half of row 1 select time column 1 presents a high voltage, for example +1.6 volts. To produce the OFF pixel at row 2 column 1, during row 2 select time, the column 1 driver presents a high voltage followed by a low voltage. To produce the ON pixel in row 3 column 1, at row 3 select time the column 1 driver begins with a low signal and moves to a high signal. The reverse occurs for row 4 and the reverse again occurs for row 5.
As shown in FIG. 4c, there is no change in voltage seen by the pixel in row 1 column 1 between the end of row 2 select time and the beginning of row 3 select time. Likewise, at the transition from row 3 to row 4 and from row 4 to row 5 no voltage change is seen by the pixel in row 1 column 1. The frequency of reversal during this unselected time for the pixel in row 1 column 1 is the same as the frequency of transition from one row time to the next. For the above example, this is 5 kHz.
Column 2 of FIG. 4c is to have all ON pixels. Therefore the column 2 driver provides a low voltage followed by a high voltage for every pixel. Thus the pixels in column 2 see two reversals for every row time. For the above example, the reversal frequency is 10 kHz. The column driver changes voltage at a frequency of 10 kHz while a column driver applying a checkerboard pattern changes voltage at a frequency of 5 kHz. Thus, the Hughes method reversal for a column driver rather than the 200:1 variation of the method of FIG. 3.
Since displays generally do not present a checkerboard pattern but have large areas of one color, a column driver for the Hughes device will on the average operate at a frequency close to this maximum frequency. Current for driving one pixel is determined by the formula EQU I=CVF
where I is current, C is capacitance of the pixel (the liquid crystal material behaves much like a capacitor as it switches polarity), V is the peak voltage difference across the pixel and F is frequency of the driving voltage. Current for driving the total display is determined by adding current for driving the individual pixels. Therefore for the Hughes method for which frequency is close to 10 kHz the current for driving the device will be consistently at the high end and therefore the device will require fairly high power. For the case of square wave pulses, power follows the formula EQU P=CV.sup.2 F
It is desirable to use a method of driving the device which requires lower current and therefore uses lower power.
In drive methods where frequency of pixel voltage polarity reversal varies by even as much as a 2:1 ratio from one part of an image to another depending upon the pattern, a phenomenon called cross-talk will occur, in which the light background in a solid region directly adjacent a checkerboard patterned region is darker than the background in the checkerboard region. Because the wave form seen by a pixel (see waveforms in FIGS. 3 and 4c, for example) is somewhat rounded at the corners, at a higher frequency the RMS value of voltage seen by the pixel will be somewhat lower than the RMS value at a lower frequency. Since the voltage difference across an ON pixel is only about 5% higher than across an OFF pixel, a 1% variation in RMS voltage is a 20% variation in the voltage difference between ON and OFF, thus the pixel color is extremely sensitive to these small voltage changes. Therefore, it is desirable that frequency of voltage polarity reversal be near constant for all image patterns in order to avoid cross-talk.